Comparing with conventional display device, an Organic Light Emitting Diode (OLED), as an electrical-current type light emitting device, is being more and more widely applied in a field of high performance display, due to its advantages of self-emission, quick response, wide view angles, and feasibility of being fabricated on a flexible base. According to the manners to be driven, OLED display devices may be divided into two categories, i.e., a Passive Matrix Driving OLED (PMOLED) and an Active Matrix Driving OLED (AMOLED). The AMOLED has advantages of low production costs, high response speeds, power-savings, capability of being used for direct-current driving in a portable device, and a wide range of operational temperature, and thus the AMOLED display is expected to become a next generation flat panel display in place of a Liquid Crystal Display (LCD).
The current OLED is driven by a Drive Thin Film Transistor (DTFT), which is typically a P-type switch transistor. A gate electrode of the DTFT is connected to a data input terminal Vdata, a source electrode thereof is connected to an input terminal VDD of a constant voltage source, and a drain electrode thereof is connected to the OLED. Because a voltage difference VGS is generated between the voltage VDD at the source electrode and the voltage Vdata at the gate electrode, the OLED connected to the drain electrode of the DTFT is turned on, and a current for driving the OLED is IOLED=K(VGS−Vth)2, wherein Vth is a threshold voltage of the DTFT, and K is a constant. From the above equation of the current for driving the OLED, it can be seen that, the threshold voltage Vth of the DTFT may affect the current IOLED for driving the OLED flowing through the OLED, and due to facts such as errors in the manufacturing processes, and aging of devices, the threshold voltage Vth of the DTFT in each pixel unit would drift, thus causing deviation of the current for driving the OLED and adversely affecting display quality. Additionally, with rapid developments of the display technique, display devices with touch functions are increasingly popular due to their merits such as visualized operations. In a type of the conventional OLED display devices, a cathode of an OLED of OLED display device further functions as a touch electrode, so as to reduce an overall thickness of the display device. In case that the cathode functions as the touch electrode, a touch driving signal is required to be applied to the cathode. However, a voltage at the touch driving signal fluctuates with time, such that the current flowing through the OLED is changed and a luminance of the OLED varies with the time, which adversely affects the display quality.